Devices for imaging body cavities or passages in vivo are known in the art and include endoscopes and autonomous encapsulated cameras. Endoscopes are flexible or rigid tubes that pass into the body through an orifice or surgical opening, typically into the esophagus via the mouth or into the colon via the rectum. An image is formed at the distal end using a lens and transmitted to the proximal end, outside the body, either by a lens-relay system or by a coherent fiber-optic bundle. A conceptually similar instrument might record an image electronically at the distal end, for example using a CCD or CMOS array, and transfer the image data as an electrical signal to the proximal end through a cable. Endoscopes allow a physician control over the field of view and are well-accepted diagnostic tools. However, they do have a number of limitations, present risks to the patient, are invasive and uncomfortable for the patient, and their cost restricts their application as routine health-screening tools.
Because of the difficulty traversing a convoluted passage, endoscopes cannot reach the majority of the small intestine and special techniques and precautions, that add cost, are required to reach the entirety of the colon. Endoscopic risks include the possible perforation of the bodily organs traversed and complications arising from anesthesia. Moreover, a trade-off must be made between patient pain during the procedure and the health risks and post-procedural down time associated with anesthesia. Endoscopies are necessarily inpatient services that involve a significant amount of time from clinicians and thus are costly.
An alternative in vivo image sensor that addresses many of these problems is capsule endoscope. A camera is housed in a swallowable capsule, along with a radio transmitter for transmitting data, primarily comprising images recorded by the digital camera, to a base-station receiver or transceiver and data recorder outside the body. The capsule may also include a radio receiver for receiving instructions or other data from a base-station transmitter. Instead of radio-frequency transmission, lower-frequency electromagnetic signals may be used. Power may be supplied inductively from an external inductor to an internal inductor within the capsule or from a battery within the capsule. The wireless-based capsule camera system will require a patient to wear a wireless transceiver and data recorder to receive and record the captured images. The capsule camera may stay in the body for over ten hours. Therefore, the patient may have to wear the wireless data receiver pack for extended hours which may be uncomfortable.
An autonomous capsule camera system with on-board data storage was disclosed in the U.S. Pat. No. 7,983,458, entitled “In Vivo Autonomous Camera with On-Board Data Storage or Digital Wireless Transmission in Regulatory Approved Band,” granted on Jul. 19, 2011. The capsule camera with on-board storage archives the captured images in on-board non-volatile memory. The capsule camera is retrieved upon its exiting from the human body. The images stored in the non-volatile memory of the retrieved capsule camera are then accessed through an output port on in the capsule camera. The images can be processed and displayed on viewing station and examined by diagnostician.
The capsule camera system typically consists of an optical lens or lens system, and a light sensing element. The light sensing element is based on integrated-circuit sensor fabricated through various manufacturing process, such as CMOS (complementary metal-oxide semiconductor) or CCD (charge-coupled device) processes. CMOS image sensors are becoming more popular and have been used extensively today in various digital imaging applications. The light sensing devices traditionally have light sensing elements, called pixels, arranged into one-dimensional (one row) or two-dimensional (many rows and columns) arrays. The pixel array is aligned with the image formed by the associated optical lens system and positioned within the focus depth of the optical system. Each pixel provides an electrical output corresponding to the incident light to which the pixel is exposed.
For the capsule camera application, the capsule device has to travel in the human body for an extended period of time. Furthermore, the capsule device usually is powered by batteries. During the course of imaging the gastrointestinal track inside the human body, a capsule camera may have to capture tens of thousands of images. The image sensor is one of the major power consuming devices. Therefore, power consumption of such CMOS image sensors is an important factor that limits the lifetime of such system, if the image sensor is powered by disposable batteries. If it is powered by rechargeable batteries, the power consumption will determine the usage time between two charges. As a result, it is desirable to reduce the power consumption of the image sensor.
A capsule camera system typically consists of one or more CMOS image sensors, LED light sources, image processing ASIC (application specific integrated circuit) and other components. In order to keep the capsule camera easily swallowable, the physical size of the capsule camera becomes very limited. As a result, the batteries are usually very small and the power consumption of all components inside the capsule camera, including the image sensor becomes a critical issue. Therefore, it is desirable to develop an image sensor with power saving control to extend the battery life.